In terms of how big your platform needs to be to support a specific number of apps, that will depend heavily on your business.
In my experience, new teams supporting 500-2500 “app instances” will have 4-6 team members.
When you get beyond 5K / 10K instances, I generally see a need to separate functions (each with 6-20 people). Again, at this enterprise scale, it really depends what you consider “the platform” and the work they’re doing.